The present invention relates to induction melting systems that use magnetic induction to heat a crucible in which metal can be melted and held in the molten state by heat transfer from the crucible.
Induction melting systems gain popularity as the most environmentally clean and reasonably efficient method of melting metal. In the induction melting furnace 1 shown in FIG. 1, the electromagnetic field produced by AC current in coil 2 surrounding a crucible 3 couples with conductive materials 4 inside the crucible and induces eddy currents 5, which in turn heat the metal. As indicated in FIG. 1, the arrows associated with coil 2 generally represent the direction of current flow in the coil, whereas the arrows associated with eddy currents 5 generally indicate the opposing direction of induced current flow in the conductive materials. Variable high frequency AC (typically 100 to 10,000 Hz) current is generated in a power supply or in a power converter 6 and supplied to coil 2. The converter 6, typically but not necessarily, consists of an AC-to-DC rectifier 7, a DC-to-AC inverter 8, and a set of capacitors 9, which, together with the induction coil, form a resonance loop. Other forms of power supplies, including motors-generators, pulse-width modulated (PWM) inverters, etc., can be used.
As shown in FIG. 2, the magnetic field causes load current 10 to flow on the outside cylindrical surface of the conductive material, and coil current 11 to flow on the inner surface of the coil conductor as shown in FIG. 2. The crucible 3 in a typical furnace is made from ceramic material and usually is not electrically conductive. The efficiency of the furnace is computed by the formula:                     η        =                  1                      1            +                                                            D                  1                                                  D                  2                                            ·                                                ρ                  1                                                  ρ                  2                                            ·                                                Δ                  2                                                  Δ                  1                                                                                        Equation        ⁢                  xe2x80x83                ⁢                  (          1          )                    
where
xcex7=furnace efficiency
D1=coil inner diameter
D2=load outer diameter
xcfx811=resistivity of coil winding material (copper)
xcfx812=resistivity of load (melt)
xcex941=current depth of penetration in copper winding; and
xcex942=current depth of penetration in load (melt).
The depth of current penetration (xcex94) is a function of a material""s properties as determined by the formula:                     Δ        =                  k          ·                                    ρ                              f                ·                μ                                                                        Equation        ⁢                  xe2x80x83                ⁢                  (          2          )                    
where:
xcfx81=resistivity in ohmxc2x7meters;
f=frequency in Hertz;
xcexc=magnetic permeability (dimensionless relative value);
xcex94=depth of penetration in meters.
The constant, 503, in Equation (2) is dimensionless.
Because current does not penetrate deep into the low resistivity copper material of the coil, the typical coil efficiency is about 80 percent when the molten material is iron. Furnaces melting low resistivity materials such as aluminum, (with a typical resistivity value of 2.6xc3x9710xe2x88x928 ohmxc2x7meters), magnesium or copper alloys have an even lower efficiency of about 65 percent. Because of significant heating due to electrical losses, the induction coil is water-cooledxe2x80x94that is, the coil is made of copper tubes 12 and a water-based coolant is passed through these tubes. The presence of water represents an additional danger when melting aluminum and magnesium and their alloys. In case of crucible rupture, water may get into molten aluminum and a violent chemical reaction may take place in which the aluminum combines with oxygen in the water (H2O), releasing free hydrogen which may cause an explosion. Contact between water and magnesium may similarly result in an explosion and fire. Extreme caution is taken when aluminum or magnesium is melted in conventional water-cooled furnaces.
Often, aluminum scrap is melted in gas-fired furnaces of a sort that are referred to as xe2x80x9cstack furnaces.xe2x80x9d As shown in FIG. 3, a stack furnace 19 consists of two chambers, a dry chamber 20 and a wet chamber 21. The scrap 18 is loaded using a charge transfer bucket 22 that dumps the scrap into the dry chamber 20 as indicated by the arrows in FIG. 3. The scrap is melted by the flame from a gas burner 23. Molten metal runs from a bottom spout 24 of the dry chamber 20 into a bath 25 in the wet chamber 21 where additional heating is provided by a second gas burner 26.
An object of the present invention is to improve the efficiency of an induction furnace by increasing the resistance of the load by using as the load a crucible made of a high temperature electrically conductive material or a high temperature material with high magnetic permeability. It is another object of the present invention to improve the efficiency of an induction furnace by reducing the resistance of the induction coil by using as the coil a cable wound of multiple copper conductors that are isolated from each other. It is still another object of the invention to properly select operating frequencies to yield optimum efficiency of an induction furnace.
It is a further object of the present invention to provide a high efficiency induction melting system with a furnace and power supply that do not use water-cooling and can be efficiently air-cooled. A further objective of the present invention is to use the high efficiency induction melting system of the present invention to melt metal from scrap, cast molds, and provide a continuous source of molten metal for processing, in a manner that is integrated with the induction melting system.
In its broad aspects, the present invention is an induction furnace that is used for melting a metal charge. The furnace has a crucible formed substantially from a material having a high electrical resistivity or high magnetic permeability, preferably a silicon carbide or a high permeability steel. At least one induction coil surrounds the crucible. The coil consists of a cable wound of a plurality of conductors isolated one from the other. An isolation sleeve electrically and thermally insulates the crucible from the at least one induction coil. Preferably, the isolation sleeve is a composite ceramic material, such as an air-bubbled ceramic between two layers of ceramic.
Copper is especially preferred for the conductors, because of its combination of reasonably high electrical conductivity and reasonably high melting point. An especially preferred form of the cable is Litz wire or litzendraht, in which the individual isolated conductors are woven together in such a way that each conductor successively takes all possible positions in the cross section of the cable, so as to minimize skin effect and high-frequency resistance and distribute the electrical power evenly among the conductors.
In another aspect, the present invention is an induction melting system that is used for melting a metal charge. The system has at least one power supply. The crucible that holds the metal charge is formed substantially from a material having a high electrical resistivity or high magnetic permeability, preferably a silicon carbide or a high permeability steel. At least one induction coil surrounds the crucible. The coil consists of a cable wound of a large number of copper conductors isolated one from the other. An isolation sleeve electrically and thermally insulates the crucible from the at least one induction coil. Preferably, the isolation sleeve is a composite ceramic material, such as an air-bubbled ceramic between two layers of ceramic. Preferably, the induction melting system is air-cooled from a single source of air that sequentially cools components of the power supply and the coil. The metal charge is placed in the crucible. Current is supplied from the at least one power supply to the at least one coil to heat the crucible inductively. Heat is transferred by conduction and/or radiation from the crucible to the metal charge, and melts the charge.
In another aspect, the present invention is an induction melting system for separating metal from scrap metal that contains heavy metal inclusions. The system includes at least one power supply. A dry chamber induction furnace receives and heats the scrap metal. The dry chamber induction furnace includes a crucible for holding the scrap metal. The crucible is formed substantially from a material having a high electrical resistivity or high magnetic permeability, preferably a silicon carbide or a high permeability steel. At least one induction coil surrounds the crucible. The coil consists of a cable wound of multiple conductors, preferably of a magnitude of copper conductors, isolated one from the other. An isolation sleeve electrically and thermally insulates the crucible from the at least one induction coil. Preferably, the isolation sleeve is a composite ceramic material, such as an air-bubbled ceramic between two layers of ceramic. The dry chamber induction furnace includes a means for run out of the molten metal from the furnace, preferably by a trough in the bottom of the furnace. A wet chamber induction furnace receives molten metal by a means for run out from the dry chamber furnace. The wet chamber furnace has a crucible similarly formed from a material of high electrical resistivity or high permeability as the crucible for the dry chamber furnace, at least one induction coil similarly formed as the coil for the dry chamber furnace, and an isolation sleeve similarly situated and formed as for the dry chamber furnace""s sleeve. The induction melting system also includes a means for removal of the heavy metal inclusions from the dry furnace induction chamber, preferably by a hinged bottom that can be opened to eject the inclusions. The lid of the dry chamber furnace can include a duct for exhausting fumes created by melting metal in the dry chamber furnace""s crucible. A vibratory conveyor can be used to place the scrap metal into the dry furnace""s conveyor. Additional wet chamber induction furnaces can be provided with transfer means, preferably a launder system, to selectively transfer the molten metal from the dry chamber furnace to any one of the wet chamber furnaces. Preferably, either the dry chamber or wet chamber furnace is, or both furnaces are, air-cooled from a single source of air that sequentially cools components of the at least one power supply and the at least one induction coil associated with either the dry chamber or wet chamber furnace, or both furnaces. Metal scrap is placed in the dry chamber crucible of the dry chamber induction furnace. Current is supplied from the at least one power supply to the at least one induction coil surrounding the dry chamber crucible to inductively heat the crucible. Heat is transferred from the crucible to the metal scrap, which produces a molten metal that runs out of the dry chamber crucible and selectively into one of the wet chamber crucibles of the wet chamber induction furnaces. Current is supplied from the at least one power supply to the at least one induction coil surrounding appropriate ones of the wet chamber crucibles to inductively heat the crucibles. Heat is transferred from the crucibles to the molten metal in the crucibles. One or more of the wet chamber crucibles can be removed from their associated wet chamber induction furnaces.
In another aspect, the present invention is an induction furnace for casting a mold from a molten metal. The system has at least one power supply. A sealed crucible holds and heats the molten metal. The crucible is formed substantially from a material having a high electrical resistivity or high magnetic permeability, preferably a silicon carbide or a high permeability steel. At least one induction coil surrounds the crucible. The coil consists of a cable wound of a magnitude of copper conductors isolated one from the other. An isolation sleeve electrically and thermally insulates the crucible from the at least one induction coil. Preferably, the isolation sleeve is a composite ceramic material, such as an air-bubbled ceramic between two layers of ceramic. A suitable but not limiting selection for the ceramic compositions is an alumina or silica based ceramic. A tube, preferably with a flanged end external to the crucible, penetrates the seal of the crucible and is partially immersed in the molten metal bath. A mold is aligned on top of the flanged end of the tube so that its gate is coincident with the opening in the tube. A port is provided in the sealed crucible for the connection of a supply of controlled pressurized gas to the interior of the crucible. Preferably, the induction furnace is air-cooled from a single source of air that sequentially cools components of the power supply and the coil. Molten metal is placed inside the crucible and the crucible is sealed. Current is supplied from the at least one power supply to the at least one coil to inductively heat the crucible. Heat is transferred from the crucible to the molten metal to keep the metal molten. Pressurized gas is injected into the sealed chamber via the gas port to pressurize the interior of the crucible and force molten metal through the tube and into the mold cavities. When the mold is filled with molten metal, the interior of the crucible is depressurized and the mold is removed from the flanged end of the tube.
In still another aspect, the present invention is an induction melting system for providing a continuous supply of molten metal. The system has at least one power supply. A sealed crucible holds and heats the molten metal. The crucible is formed substantially from a material having a high electrical resistivity or high magnetic permeability, preferably a silicon carbide or a high permeability steel. At least one induction coil surrounds the crucible. The coil consists of a cable wound of a magnitude of copper conductors isolated one from the other. An isolation sleeve electrically and thermally insulates the crucible from the at least one induction coil. Preferably, the isolation sleeve is a composite ceramic material, such as an air-bubbled ceramic between two layers of ceramic. An inlet conduit has a receiver end external to the sealed crucible and an opposing end internal to the sealed crucible. The opposing end is immersed in the molten metal bath. An outlet conduit protrudes through the sealed crucible and has one end immersed in the molten metal bath and an opposing exit end that is external to the crucible. A port is provided in the sealed crucible for the connection of a supply of controlled pressurized gas to the interior of the crucible. Preferably, the induction furnace is air-cooled from a single source of air that sequentially cools components of the power supply and the coil. Furnace feed material is continuously supplied to the crucible at the receiver end of the inlet conduit. Feed material is continuously heated by heat transfer from the crucible, which is inductively heated by the at least one induction coil surrounding the crucible. Pressurized gas is injected into the sealed chamber via the port to pressurize the interior of the crucible and continuously force molten metal through the outlet conduit to its exit end. The outlet conduit may be a siphon, which can maintain a continuous flow of molten metal from the crucible without the requirement for maintaining a continuous positive pressure in the interior of the crucible. A gas port may be provided in the siphonal outlet conduit for the injection of a gas into the outlet conduit to break the continuous flow of molten metal.
These and other aspects of the invention will be apparent from the following description and the appended claims.